Because it is already in the game, but not as a weapon, it's the graphic for the level 1 and 2 melee Limit Breaks 'Braver' and 'Blade Dance' (Braver naturally was Cloud's basic Limit Break in FFVII so it made sense to have the Buster Sword appear there).
http://vignette2.wikia.nocookie.net/...20150707122640
Also remember that Limit Breaks have been in ARR since launch, where as DRK was only added in
Heavensward and, given SE's reluctance to retcon fanservice gear that was a part of earlier events for new Jobs (aka, the FFXIII weapons), I don't see this happening I'm afraid.